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Exploration of the electronic structure of dendrimerlike acetylene-bridged oligothiophenes by correlating Raman spectroscopy, electrochemistry, and theory

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Exploration of the electronic structure of dendrimerlike acetylene-bridged oligothiophenes by correlating Raman spectroscopy, electrochemistry, and theory
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  Explorationoftheelectronicstructureofdendrimerlikeacetylene-bridgedoligothiophenesbycorrelatingRamanspectroscopy,electrochemistry,andtheory Juan Casado a)  Department of Physical Chemistry, University of Ma´ laga, Campus de Teatı`nos s/n, Ma´ laga 29071, Spain Ted M. Pappenfus  Division of Science and Mathematics, University of Minnesota, Morris, Minnesota 56267  Kent R. Mann  Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455 Vı´ctor Herna´ndez and Juan T. Lo´pez Navarrete b)  Department of Physical Chemistry, University of Ma´ laga, Campus de Teatinos s/n, Ma´ laga 29071, Spain ͑ Received 12 November 2003; accepted 2 April 2004 ͒ A series of radial thiophene-based structures consisting of a central benzene or thiophene ringsurrounded by acetylene-bridged terthienyl arms has been investigated by physical and theoreticalmethods. Fourier transform Raman spectroscopy of the neutral solids shows that the terthiophenearms are weakly coupled across the core ͑ benzene plus acetylene groups ͒ likely due tocross-conjugation or meta-conjugation effects that may prevent full delocalization. By increasingthe number of arms around the central ring, the electronic structure of the molecules seems to beaffected only at the core, whereas the outer terthiophene arms remain almost unaltered. Ramanspectroelectrochemistry and quantum chemical calculations provide further insight into the chargedelocalization of the oxidized species. There is no evidence to suggest that these oxidized forms,obtained upon electrochemical doping of the molecules, show charge delocalization across the core.© 2004 American Institute of Physics. ͓ DOI: 10.1063/1.1755665 ͔ I.INTRODUCTION Multichromophore dendrimers are able to transfer en-ergy rapidly and efficiently to a central core. To understandenergy flow in these molecules, one must identify the rel-evant light-absorbing and emitting units and determine theirelectronic coupling. This is relatively straightforward forwell-separated chromophores, but more difficult for conju-gated supermolecules such as conjugated dendrimers. 1 The successful use of benzene-core dendrimers in mate-rials science is well reported owing to their potential liquid-crystalline behavior and to their ability to self-assemble andto form supramolecular architectures. 2,3 The understandingof the electronic structure of these macromolecules from thestandpoint of the electronic interactions between their build-ing conjugated blocks is needed since, for example, theymainly govern the mechanisms of charge transfer from theperipheral groups to the core ͑ antennae systems ͒ , or becausethey are responsible for the mechanisms in which opticalexcitations efficiently convert into charge-separated states ͑ photonic materials in solar cells ͒ . The precise understandingof the structure-property relationships in molecular materialsare of primary importance since the optimization of theirelectronic properties will mainly depend on how the elec-tronic structure is related with the molecular structure or withthe chemical functionalization. A useful and practical strat-egy for investigating these relationships is the systematicvariation of the number of repeat units or varying substitu-tion patterns in a particular type of molecule. In this regard,it is desirable to deal with systems of conjugated moleculesof this type which allow the use of spectroscopic techniques,electrochemical methods, and quantum chemistry. 3,4 Pappenfus et al. have recently prepared a series of mol-ecules that can be viewed as models of the first generation of larger dendrimers. These molecules consist of a central ben-zene or thiophene ring surrounded by three, four or sixacetylene-bridged terthiophenes ͑ herein denoted as arms ͒ . 5 This strategy is attractive since it combines the excellent op-tical and electronic properties of oligothiophenes with thephysical properties of dendrimers. 6 ͑ a ͒ It is also noted thatthese systems could be explored as organic electroluminis-cent materials with potential applications in light-emittingdevices since they may render materials with stable amor-phous morphologies, and because of the high concentrationof functional groups ͑ hole-transporting units ͒ which may bebeneficial to the physical performance of the potentialdevice. 6 ͑ b ͒ Furthermore, charge transport or energy flow fromthe peripheral groups to the core are many times associatedwith charged separated states. In a first approach, one canreasonably model these states with the analysis of the fullyionized ͑ anionic or cationic ͒ species. Accordingly, to achievemolecular materials with enhanced hole-transporting proper-ties or improved electrochromic behavior, it is necessary to a ͒ Electronic mail: casado@uma.es b ͒ Author to whom correspondence should be addressed. Electronic mail:teodomiro@uma.esJOURNAL OF CHEMICAL PHYSICS VOLUME 120, NUMBER 24 22 JUNE 2004 118740021-9606/2004/120(24)/11874/8/$22.00 © 2004 American Institute of Physics Downloaded 11 Jun 2004 to 150.214.50.234. Redistribution subject to AIP license or copyright, see http://jcp.aip.org/jcp/copyright.jsp  understand the electronic structure of the charged species.In this work, we explore the electronic structure of theneutral and the oxidized species of a series of radial oligoth-iophenes with different substitution patterns. This study fo-cuses on the interactions between the peripheral groups andthe core, and on the role played by the acetylene groups bymeans of spectroscopy, electrochemistry, and quantum chem-istry. FT–Raman spectroscopy is the guide method used inthis study to analyze the electronic features since it is per-fectly suited, in combination with the effective conjugationcoordinate ͑ ECC ͒ theory, 7 for dealing with the evaluation of the ␲  -conjugational properties of virtually any polyconju-gated molecule. The methodology of the work will consistfirst on the assignment of the Raman bands both in the neu-tral and in the charged compounds. Second, we will analyzethe wave number changes of the Raman lines as a function of the substitution pattern, i.e., those mostly related, within theframework of the ECC model, with the degree of involve-ment of each moiety in the overall ␲  -electron conjugation. Inall the cases theoretical calculations on model compoundswill be carried out in order to support the experimental find-ings. II.EXPERIMENTALANDTHEORETICALDETAILS The synthesis of these acetylene bridged oligothiopheneshas been reported elsewhere. 5 A representative chemicalstructure for the molecules is displayed in Fig. 1. FT–Ramanspectra were measured using an FT–Raman accesory kit ͑ FRA/106-S ͒ of a Bruker Equinox 55 FT–IR interferometer.A continuous-wave Nd–YAG laser working at 1064 nm wasemployed for Raman excitation. A germanium detector oper-ating at liquid nitrogen temperature was used. Raman scat-tering radiation was collected in a back-scattering configura-tion with a standard spectral resolution of 4 cm Ϫ 1 . In order toavoid possible damage to the samples upon laser radiation,especially regarding the oxidized species, the laser beam wasloosely focused on the sample and its power was kept at alevel lower than 100 mW and 500 scans were averaged foreach spectrum.Full geometry optimizations were performed in theframework of the density functional theory by means of theB3LYP functional, using the A.7 revision of the GAUSSIAN 98 program package running on a SGI Origin 2000 computer. 8,9 The 3-21G * basis set was chosen to reduce the high dimen-sion of the problem. 8,10 Due to the large molecular size of thecompounds studied in this work, quantum chemical calcula-tions for the neutral systems were carried out on two modelsystems built up from a central benzene ring surrounded bythree and six acetylene thiophene groups ͓ Ph ͑ AlT) 3 andPh ͑ AlT) 6 ]. A benzene ring surrounded by two ␣  -acetylene, ␣  Ј -phenyl terthiophene groups in para positions, denoted as  para Ph ͑ A3TPh) 2 , was used to model the effects of ioniza-tion. As usual, the effect of the doping was mimicked byconsidering the radical cationic or dicationic species of  para Ph ͑ A3TPh) 2 . Geometry optimizations were performed onisolated entities in the vacuum. The neutral and dicationicspecies were treated as closed-shell systems ͑ B3LYP proce-dure ͒ , while for the radical cations ͑ open-shell systems ͒ op-timizations were carried out using spin unrestricted wavefunctions ͑ UB3LYP ͒ . The model systems were consideredwithout butyl groups, being confident that these alkyl sidechains have minimal effects on the conjugated path. All ge-ometry parameters ͑ i.e., bond lengths, bond angles, and di-hedral angles ͒ were allowed to vary independently along themolecular optimizations. For the ionized species, negativecounterions were not considered.Electrochemical studies ͑ cyclic voltammograms ͒ wereobtained with a BAS 100B electrochemical analyzer using aglassy carbon working electrode. For the Raman spectroelec-trochemical studies, thin solid films were obtained by castingon a Pt working electrode from a CH 2 Cl 2 dispersion of thesolid. The film of the compound on the Pt working electrodewas then immersed in a 0.1 M tetrabutyl ammonium FIG. 1. Representative chemical structures and nomenclature used for thestudied compounds. 11875J. Chem. Phys., Vol. 120, No. 24, 22 June 2004 Electronic structure of acetylene-bridged oligothiophenes Downloaded 11 Jun 2004 to 150.214.50.234. Redistribution subject to AIP license or copyright, see http://jcp.aip.org/jcp/copyright.jsp  hexafluoro phosphate (TBAPF 6 ) solution in acetonitrile. A1 ϫ 1 cm 2 Pt electrode and a Ag/AgCl electrode were used asthe auxiliary and reference electrodes, respectively. Oxida-tion of the films, using a chronoamperometric technique for50 s, was run up to 2 V to avoid for solid state hysteresisprocesses that could diminish the real potential in the solid–electrode interface, thus ensuring for the complete oxidationof the thin films. III.VIBRATIONALCONSIDERATIONSA.ECCtheory,Ramanspectraand ␲  -electronconjugation The Raman spectrum of a given molecule, or a series of molecules, can be theoretically explored by group theory onthe basis of the optical selection rules for the Raman scatter-ing process. It is well known from many x-ray diffractionstudies that oligothiophenes usually display a full planar ornearly planar molecular conformation in solid state. 4 ͑ b ͒ Therefore, one may reasonably consider that in solid phase,the molecules subject of the present investigation are fully oralmost planar. Under this assumption, for example,Ph ͑ A3TPh) 4 belongs to the D 2 h symmetry point group and,since it has 240 atoms ͑ N ͒ , there exists 714 modes ͑ 3N-6 ͒ of vibration of which 357 are Raman active. More precisely, thenormal modes distribution among the various symmetry spe-cies of the D 2 h point group is the following: 120(  A g ) ϩ 58(  B 1 g ) ϩ 119(  B 2 g ) ϩ 60(  B 3 g ).The experimental solid state Raman spectrum of Ph ͑ A3TPh) 4 in Fig. 2 displays however only 10–12 well-resolved bands, which is in principle unexpected. This con-tradiction can be justified as follows: ͑ a ͒ when Franck–Condon scattering is predominant, totally symmetric modes(  A g ) are found to be selectively enhanced, while B g (  B 1 g ,  B 2 g , and B 3 g ) modes become weak or almost undetectable; ͑ b ͒ there exists a nonuniform distribution of Raman intensityamong the different totally symmetric modes. This phenom-enon can be accounted for by the existence of a very largeelectron–phonon coupling between the electronic structureof the system and the nuclear motions for a given vibration ͑ generally the normal mode that mostly mimics the geo-metrical evolution from the ground electronic state to thefirst excited state ͒ . Consequently, as for the ␣  , ␣  Ј -linked oli-gothiophenes ͑ or any other five-member ring polyconjugatedmolecule ͒ , this vibration consists of a collective C v C/C–Cstretching mode in which all the C v C and C–C bondsshorten and lengthen in phase, respectively. It is usuallytermed as the ECC mode in the effective conjugation coor-dinate theory. 7 Furthermore, the electron–phonon couplinggives rise to a selective enhancement of the C v C/C–Cstretchings of the conjugated path.For low band-gap polyconjugated oligomers, the ECCmode strongly couples with ␲  electron delocalization. Thus,the C v C/C–C stretching vibrations that compose the ECCmode give information about the ␲  -electron conjugation. Atthe level of the molecular geometry parameters an incrementof  ␲  -electron conjugation or ␲  -electron delocalization in agiven oligothiophene series can be described by the loweringof the overall BLA parameter ͑ i.e., the average differencebetween the conjugated C v C and C–C bond lengths ͒ be-cause of the continuous weakening of the double bonds andthe strengthening of the single bonds. This is a consequenceof the loss of aromaticity of the five-membered rings at theexpense of gaining quinoid character within the structure. Atthe level of the molecular force field which determines thewave numbers of the Raman lines, the increasing contribu- FIG. 2. FT–Raman spectra of  ͑ a ͒ Ph ͑ A3TPh) 3 , ͑ b ͒ Ph ͑ A3TPh) 4 , ͑ c ͒ Th ͑ A3TPh) 4 , and ͑ d ͒ Ph ͑ A3TPh) 6 . The spectrum labeled as ͑ e ͒ corresponds to thecalculated B3LYP/3-21G * Raman spectrum of the para Ph ͑ A3TPh) 2 model in neutral state. 11876 J. Chem. Phys., Vol. 120, No. 24, 22 June 2004 Casado et al. Downloaded 11 Jun 2004 to 150.214.50.234. Redistribution subject to AIP license or copyright, see http://jcp.aip.org/jcp/copyright.jsp  tion of the quinoid form to the electronic structure of thesystem gives rise to a lower value or softening of the forceconstants associated to the ECC mode. Consequently the in-creased ␲  -electron conjugation along a homologous series of compounds can be followed by a wave number downshift of some of the strong C v C/C–C stretching vibrations appear-ing in the Raman spectrum. 7 B.AssignmentoftheRamanspectra Figure 2 displays the FT–Raman spectra of the four ra-dially branched oligothiophenes. For comparison purposes,the Raman spectrum of 3T ͑ terthiophene with the two inner-most ␤  positions substituted with butyl groups ͒ and the spec-trum of Ph3TPh are shown in Fig. 3.A well resolved Raman peak is recorded in each casearound 2200 cm Ϫ 1 due to the fully in-phase C w C stretchingvibration, ␯  s (C w C), measured at 2197 cm Ϫ 1 inPh ͑ A3TPh) 3 , 2181 cm Ϫ 1 in Ph ͑ A3TPh) 4 , 2174 cm Ϫ 1 inPh ͑ A3TPh) 6 , and 2170 cm Ϫ 1 in Th ͑ A3TPh) 4 . 11 Thesebands are quite intense, in some cases even more than thoseat 1460 cm Ϫ 1 due to the thienyl vibrations. Owing to its sp hybridization, acetylide bonds can be viewed as weakelectron-accepting or withdrawing groups relative to their sp 2 counterparts. According to the theoretical calculationsperformed for the model systems, the triple C w C bond ac-cumulates a total negative charge ͑ sum of the Mu¨llikenatomic charges of the two sp carbon atoms ͒ of  Ϫ 0.1730 e inPh ͑ A1T) 6 versus Ϫ 0.1172 e in Ph ͑ A1T) 3 . At the same time,the C w C bond distances lengthen by 0.001 Å upon hexasubstitution. One may argue that, upon increasing branchingof the core, the electron-accepting acetylene bridges inducean increasing confinement of the ␲  electrons in their sur-rounding and, likely due to the repulsion of the accumulatedcharge, loosing triple bond character and shifting its ␯  s (C w C) associated Raman to lower wave numbers. 11 InTh ͑ A3TPh) 4 with a much more polarizable thienyl core, theattraction of the ␲  electrons by the triple bonds should beeven more pronounced.The bands associated with the skeletal C v C stretchingvibrations, ␯  ͑ C v C ͒ , of the phenyl groups appear in the1600–1570 cm Ϫ 1 spectral region. 11 The ␯  ͑ C v C ͒ stretchingof the phenyl end caps of Ph3TPh has been previously as-signed to the bands at 1597 cm Ϫ 1 , which clearly correlateswith the lines at 1597 cm Ϫ 1 in Ph ͑ A3TPh) 3 andPh ͑ A3TPh) 4 , at 1599 cm Ϫ 1 in Ph ͑ A3TPh) 6 and at 1598cm Ϫ 1 in Th ͑ A3TPh) 4 . 12 Quite small wave number differ-ences for this Raman scattering are observed among the se-ries of compounds which is consistent with the negligiblechanges of the electronic and molecular structures of theseouter phenyl groups in the series.Bands at 1579 cm Ϫ 1 in Ph ͑ A3TPh) 3 , 1584 cm Ϫ 1 inPh ͑ A3TPh) 4 and 1574 cm Ϫ 1 in Ph ͑ A3TPh) 6 are likely dueto ␯  ͑ C v C ͒ stretching of the central phenyl ring. 11 The low-est wave number corresponds precisely to the most radiallybranched compound which reveals that this is the mostelectron-deficient phenyl ring in the series. B3LYP/3-21G * Mu¨lliken atomic charges from calculations also provide evi-dence of the increased positive charge over the central phe-nyl ring with the increasing number of peripherical electron-withdrawing acetylenic groups: ϩ 0.09344 e in Ph ͑ A1T) 3 versus ϩ 0.18761 e in Ph ͑ A1T) 6 . Additional data were de-rived from a molecule related to Ph ͑ A3TPh) 3 , but lacking of the acetylene bridges: its solid-state Raman spectrum dis-plays a band at 1589 cm Ϫ 1 associated to ␯  ͑ C v C ͒ of its cen-tral phenyl core, whereas it is measured at 1579 cm Ϫ 1 inPh ͑ A3TPh) 3 . 11,13 Most Raman bands corresponding to thienyl vibrationsappear in the 1550–1000 cm Ϫ 1 spectral region. 7,12,14 Thecharacteristic Raman spectral features of the 3T moiety iseasily recognized in the spectra of all the radial compoundsfor which they dominate the whole Raman fingerprint. Inparticular, bands at 1490, 1460, 1440, 1385 cm Ϫ 1 , and 1070–1050 cm Ϫ 1 , associated to various oligothienyl vibrationalmodes, are clearly observed.Lines at 1490 cm Ϫ 1 in Ph ͑ A3TPh) 3 , 1489 cm Ϫ 1 inPh ͑ A3TPh) 4 , 1493 cm Ϫ 1 in Ph ͑ A3TPh) 6 , and 1493 cm Ϫ 1 inTh ͑ A3TPh) 4 correspond to that usually termed as line A inthe Raman spectra of  ␣  -linked oligothiophenes and the cor-responding vibration can be described as a in-phase C v Cantisymmetric oscillation of the outermost thienyl rings of the chain. 7,12,14 As in the present compounds, the line A gen-erally shows a little dependence on the number of conjugatedunits in the chain: thus, it appears at 1503 cm Ϫ 1 in ␣  , ␣  Ј -diphenyl thiophene, 1499 cm Ϫ 1 in ␣  , ␣  Ј -diphenylbithiophene, and 1496 cm Ϫ 1 in ␣  , ␣  Ј -diphenyl terthiophene. 12 Line B is always the strongest band of the Raman spec-tra of oligothiophenes. 7,12,14 While it largely downshifts withthe increasing chain length in oligopyrroles and oligofurans,in oligothiophenes its peak position is quite little dependenton the chain length. For the molecules under study, line B,which corresponds to a in-phase C v C symmetric vibrationspreading over the whole terthienyl backbone, can be as-signed to the bands at 1461, 1458, 1462, and 1461 cm Ϫ 1 ,respectively, in Ph ͑ A3TPh) 3 , Ph ͑ A3TPh) 4 , Ph ͑ A3TPh) 6 ,and Th ͑ A3TPh) 4 . Owing to its large intensity for all thecompounds, this scattering can be reasonably assigned to theECC mode. 7,12,14 FIG. 3. FT–Raman spectra of  ͑ a ͒ 3T and ͑ b ͒ Ph3TPh. 11877J. Chem. Phys., Vol. 120, No. 24, 22 June 2004 Electronic structure of acetylene-bridged oligothiophenes Downloaded 11 Jun 2004 to 150.214.50.234. Redistribution subject to AIP license or copyright, see http://jcp.aip.org/jcp/copyright.jsp  Close to line B also the so-called line C is distinguishedat 1444, 1442, 1443, and 1441 cm Ϫ 1 , respectively, inPh ͑ A3TPh) 3 , Ph ͑ A3TPh) 4 , Ph ͑ A3TPh) 6 , andTh ͑ A3TPh) 4 . Its associated vibration can be described as theout-of-phase symmetric C v C stretching of the inner thienylrings of the chain. 7,12,14 As for Th ͑ A3TPh) 4 , the intense lineat 1410 cm Ϫ 1 can arise from a stretching mode of the centralthiophene which can be related with the Raman lines ataround 1415 cm Ϫ 1 in oligothiophenes bearing a full quinoidstructure for their thiophene rings ͑ dicationic species ͒ . In thissense, one could think in the appearance of a quinoidlikestructure for this thiophene core due again to the effect of thesurrounding acetylene groups. 15 Near to this band at around 1385 cm Ϫ 1 medium-intensescatterings are observed for all the samples. These bands areabsent for the nonbutylated terthiophenes and always appearafter alkylation of the ␤  positions of the central thiophene. 12 Therefore, it can be assigned to a vibration composed by ␯  ͑ C–C ͒ and ␤  (CH 2 ) modes of the butyl side chains exten-sively coupled to the ECC mode in a similar way as line Ddoes. Line D appears at 1055, 1050, 1052, and 1056 cm Ϫ 1 ,respectively, in Ph ͑ A3TPh) 3 , Ph ͑ A3TPh) 4 , Ph ͑ A3TPh) 6 ,and Th ͑ A3TPh) 4 and corresponds to the in-phase C–H de-formation vibration, ␦  ͑ C–H ͒ , of the aromatic CH bonds thatare extensively coupled with the dynamics of line B since theC ␤  atoms of the thiophenes in the ␯  ͑ C v C ͒ motions leads torecoil the CH bonds giving rise to a mechanical coupling andthe subsequent activity enhancement of the ␦  ͑ C–H ͒ Ramanlines. 7,12,14 C.Densityfunctionaltheorycalculations At this point of the analysis we have observed that Ra-man wave numbers arising from the outer phenyl and ter-thiophene units do not change significantly within the seriesof compounds. This seems to indicate that the arms are mini-mally affected by the branching pattern around the centralcore. Accordingly, the choice of the Ph ͑ A1T) 3 andPh ͑ A1T) 6 models in which each arm is replaced by onethiophene ring could be sufficient to simulate the structuraland electronic effects on the core. Calculations at theB3LYP/3-21G * level carried out for Ph ͑ A1T) 3 andPh ͑ A1T) 6 show negligible differences since the larger CCbond length changes found for the ␤  , ␤  Ј thienyl bonds do notexceed from 0.0003 Å, whereas the Mu¨lliken atomic chargesof the ␤  Ј atoms of the thiophene rings are also nearly thesame: ϩ 0.017262 e in Ph ͑ A1T) 3 and ϩ 0.017268 e inPh ͑ A1T) 6 . In contrast, the calculated B3LYP/3-21G * CCbond lengths for the central phenyl core of Ph ͑ A1T) 3 andPh ͑ A1T) 6 amount to 1.4066 and 1.4233 Å, respectively, andthe CC bonds connecting the acetylene bridge to the centralphenyl ring and to the thiophene arms shorten, respectively,by 0.089 and 0.036 Å in passing from Ph ͑ A1T) 3 toPh ͑ A1T) 6 . These theoretical data seemingly give support tothe progressive attenuation of electronic and structural modi-fications upon increasing branching of the central phenylcore at the level of the first thiophene rings of the model. D.UV–Viscomplementarydata The behavior of the wavelength maximun of the ␲  – ␲  * electronic absorption within the series: 400 nm inPh ͑ A3TPh) 3 , 417 nm in Ph ͑ A3TPh) 4 , 458 nm inPh ͑ A3TPh) 6 , and 402 nm in Th ͑ A3TPh) 4 represents one of the few cases where no saturation of the wavelength of thistransition with the increasing number of thiophene rings isobserved. 5 For most linear oligothiophenes the wavelengthof the maximun of the ␲  – ␲  * absorption meets saturation atthe level of 10–12 thiophene units, however we call the at-tention that in Ph ͑ A3TPh) 6 there are present up to 18thiophene rings. 16 Since, upon increasing branching, theelectronic and structural differences mainly affect to the cen-tral core, one would in principle expect that the whole posi-tion of the ␲  – ␲  * band could not depend, at least linearly, onthe number of thiophene rings of the macromolecule.We would like also to mention that the electronic spectraof neutral 3T, Ph3TPh, Ph3TASiMe 3 (SiMe 3 denotes the tri-methyl silyl group ͒ and Ph ͑ A3TPh) 3 display the maximun of their respective ␲  – ␲  * bands at 336, 384, 383, and 400 nm.This agrees with the fact that the absorbing units in thesesystems mainly consist of a phenyl–terthienyl moiety, onlyslightly affected by the presence of the core. Moreover, thehomologue molecule of Ph ͑ A3TPh) 3 without acetylenegroups shows the ␲  – ␲  * band maximun at 396 nm whichfurther reveals the minimal involvement of the triple bondsregarding the HOMO–LUMO transition placed at thephenyl–terthienyl arms.For the general discussion of the results, it is convenientto review the literature for explanations of similar spectro-scopic phenomena of acetylene-bridged compounds. In thecase of some oligothiophenes made up with terthiophenegroups bridged by a triple bond, the authors invoked theparticipation of resonance structures consisting of cumulateddouble bonds. These cumulated bonds could induce confor-mational and ␲  -electronic cloud distortions which could pre-clude for full conjugation between the two terthienylgroups. 17 But these molecules could also be viewed as cross-conjugated systems in the sense that there exist double ortriple bonds that are in conjugation but not in a lineardisposition. 18 In contrast to linear conjugated chromophores,cross-conjugated molecules generally display a reduced ␲  -electron conjugation. IV.CHARGEDSPECIES The electrochemical properties of these radiallybranched oligothiophenes have been previously investi-gated. 5 The cyclic voltammogram of Ph3TASiMe 3 in 0.1 MCH 2 Cl 2 TBAPF 6 is displayed in Fig. 4. This oligomer dis-plays much more stable oxidation processes than the previ-ously investigated Ph3TAH homologue and consisting of two one-electron processes associated to the successive gen-eration of the radical cation and dication. Figure 5 also dis-plays the cyclic voltammogram of Ph ͑ A3TPh) 3 in which thefirst and second processes at 0.93 and 1.00 V likely involvethe formation of cation radicals on each arm while the thirdprocess at 1.41 V could be assigned to the formation of thetricationic species followed by the generation of higher oxi- 11878 J. Chem. Phys., Vol. 120, No. 24, 22 June 2004 Casado et al. Downloaded 11 Jun 2004 to 150.214.50.234. Redistribution subject to AIP license or copyright, see http://jcp.aip.org/jcp/copyright.jsp
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